One aspect of the use of antibiotics in animal agriculture, which to date has received modest attention, concerns the effects of antibiotics excreted by animals upon the environment. The persistence of 7 antibiotics that are commonly used in feeds was studied in a typical agricultural soil. The antibiotics studied were bacitracin (BAC), penicillin (PEN), streptomycin (STR), tylosin (TYL), bambermycins (BAM), erythromycin (ERY), and chlortetracycline (CTC). The level incorporated into the soil with feces was 5.6 μg antibiotic/g soil. After incubation in soil for 30 days at 30°C, 44% of CTC and 23% of BAC remained; however, no detectable amounts of TYL, ERY, BAM, STR, and PEN remained. At 20°C, 88% of CTC, 33% of BAC, and 25% of ERY remained after 30 days; however, no STR, BAM, TYL, and PEN could be found. At 4°C, essentially all of the CTC, ERY, and BAM; 23% of BAC; and 40% of TYL remained after 30 days; however, PEN and STR were undetectable.